The aluminium rolling process is critical to the aluminium manufacturing industry, where precise temperature measurement is essential for ensuring high-quality product output and protecting equipment. This process produces aluminium coils through cold rolling, direct casting and continuous casting, and plays a fundamental role in determining the material’s final properties.
During this process, aluminium slabs or billets undergo either cold or hot working, depending on the desired material characteristics. Cold rolling increases the strength and hardness of the aluminium by altering its microstructure, though it also makes the material more brittle. Conversely, hot working conducted at temperatures between
One of the key challenges is the accurate temperature measurement of aluminium, mainly due to its highly reflective surface. Aluminium is notoriously difficult to measure with infrared cameras because of its low emissivity and high reflectivity in the infrared spectrum. Especially when polished or bright, aluminium’s emissivity is very low, typically ranging from
Harnessing the wedge effect
One effective way to address the emissivity challenge in temperature measurement is to focus on the point where the coil forms a wedge, creating a nearly enclosed cavity for infrared radiation. In the deepest part of this wedge, the cavity acts as a virtual blackbody, with a stable emissivity close to one. This approach, known as the wedge method, is particularly practical for measuring the temperature of polished steel strips, which typically have a low emissivity of around 0,37. The wedge naturally forms within the coiled strip making it an optimal location for accurate temperature readings.
The wedge effect explains why surfaces within a wedge-shaped structure exhibit higher apparent emissivity compared to flat surfaces. This increased emissivity results from multiple reflections of infrared radiation within the wedge, where the radiation bounces off the walls, enhancing the likelihood of absorption and re-emission. This process creates an environment that closely mimics a perfect blackbody, with an emissivity approaching 0,998, regardless of the material’s natural emissivity. This phenomenon significantly reduces the impact of emissivity variations and background radiation on temperature measurements. For the wedge effect to be effective, the depth of the wedge must significantly exceed its opening.
The Xi410 infrared camera can autonomously leverage the wedge effect despite being a long-wavelength device. The thermal imager can accurately identify the highest temperature by configuring a region of interest (ROI) in the PIXConnect software to capture the maximum temperature within the wedge of the aluminium coil. The Xi410 can then transmit thermal data directly to a programmable logic controller (PLC) via analogue outputs or digital interfaces, ensuring precise and reliable temperature monitoring in real-time.
Achieving optimal cold rolling with affordable temperature monitoring
When rolling high-grade products the decisive difference is precise temperature control. If the required temperatures are precisely maintained, the product quality can be assured and damage to the roll stand will be avoided. This application is suitable for the wedge measurement technique used at the coiler or between the roll and the product during the rolling process, which eliminates the challenges posed by low temperatures and variable emissivity, and also eliminates background reflection influences. This allows the manufacturer to implement continuous process temperature monitoring. It ensures that the system consistently tracks temperature variations throughout the production process, allowing for real-time adjustments and supporting the documentation of process and tool temperatures, providing insights into the production process and enabling closed-loop control for automated adjustments that maintain optimal conditions and prevent deviations.
The Optris long-wavelength camera can exhibit the wedge effect to accurately measure the strip temperatures in cold rolling and coiling applications. In this application, the autonomous thermal imager from the Xi Series provides multiple interface options, enabling the camera to be easily integrated into new or existing process control systems. Unlike other solutions from the competition, there is no need for special software or unique sets of wavelengths in this challenging application, as Optris infrared cameras will measure the wedge effect at an affordable price.
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